There have been widely used video cameras and electronic cameras in recent years. These cameras include solid-state image sensors such as CMOS image sensors. The solid-state image sensors each include an imaging block in which a plurality of photoelectric conversion blocks each configured by a photodiode are disposed in a two-dimensional array, and there are formed unit regions (unit pixels) each including the photodiode as a main functional part.
FIG. 12 is an explanatory diagram, exemplarily showing the configuration of an imaging block in a conventional CMOS image sensor and an equivalent circuit for one unit pixel. In the CMOS image sensor shown in FIG. 12, each unit pixel 100 includes a photoelectric conversion block configured by a photodiode 102 and three MOS transistors 103, 105, and 107 each of which converts signal charges into voltage signals to output the voltage signal (see Patent Document 1, for example).
Upon receiving light beams, signal charges (electrons) accumulated in the photodiode 102 are transferred to a floating diffusion (FD) block 104 through the electric charge transfer transistor 103 in accordance with readout pulses that are applied from a readout signal line 109 to a gate electrode of the electric charge transfer transistor 103. The FD block 104 is connected to a gate electrode of the amplifier transistor 105, and a variation in electric potential of the FD block 104 caused by the signal charges (electrons) is impedance-converted by the amplifier transistor 105 and is then outputted to a vertical signal line 15. The reset transistor 107 resets the electric potential of the FD block 104 so as to be equal to the electric potential of a power supply line 108 in accordance with a vertical reset pulse that is applied from a vertical reset line 14 to a gate electrode thereof.
The unit pixels 100 are scanned as follows, one time each in one cycle by a vertical shift register 11 as well as by a horizontal shift register 12. More specifically, when pulses are supplied from the vertical shift register 11 to one reset line 14 during a constant period of time in one cycle, the reset transistor 107 connected to the reset line is turned ON to cause the floating diffusion block 104 to be set to a high potential and to be selected. When there are selected pixels in a row corresponding to this reset line 14 by this operation, signals outputted from the respective pixels are transmitted to the corresponding vertical signal lines 15. During this constant period of time, horizontal select pulses are sequentially outputted from the horizontal shift register 12 to respective horizontal select lines 17, and signals outputted from the corresponding vertical signal lines 15 are sequentially extracted to horizontal signal lines 18 through horizontal select transistors 16, respectively. Upon completing the scanning of all the pixels in the same row, vertical select pulses are supplied to the reset line 14 in the following row so as to scan the respective pixels in this following row in the manner similar to the above. These operations are repeated to scan all the pixels in all the rows one time each during one cycle, and output signals thereof are extracted to the horizontal signal lines 18 in a time-series manner.
In recent years, there has been growing requirement for reduction in size of a solid-state image sensor for the purpose of adding camera functions to mobile apparatuses such as mobile phones. Such reduction in size of the solid-state image sensor as well as reduction in size of unit pixels for the purpose of increase in the number of pixels lead to reduction in light receiving area in one unit pixel, which deteriorates the properties of the solid-state image sensor such as the quantity of saturation signals and sensitivity.
In order to prevent such deterioration in property, there have been conventionally proposed a method of sharing photodiodes to reduce the number of transistors per unit pixel, as well as breakthroughs by optimization of the sharing method and the like (see Patent Document 2, for example).
However, since these are merely conceptual propositions, it has been difficult to preferably maintain the properties of the solid-state image sensor only by such methods.